A non-aqueous electrolyte secondary battery, particularly a lithium secondary battery has high energy density, and is therefore widely used as a battery to be used for a personal computer, a mobile phone, a portable information terminal and the like.
Non-aqueous electrolyte secondary batteries typified by the lithium secondary battery have high energy density, so that if an internal short circuit/external short circuit occurs due to damage to the battery or damage to a device using the battery, or the like, heavy current flows to generate heat. Therefore, the non-aqueous electrolyte secondary battery is demanded to secure a high level of safety by preventing heat generation that exceeds a certain level.
As means for securing the safety, a method for providing a shutdown function to prevent additional heat generation by blocking passage of ions between positive and negative electrodes by a separator in case of abnormal heat generation is commonly used. Examples of the method for causing a separator to have a shutdown function include a method in which a porous film consisting of a material that is melted at the time of abnormal heat generation is used as a separator. That is, in a battery using the separator, at the time of abnormal heat generation, the porous film is melted and made non-porous to block passage of ions, so that additional heat generation can be suppressed.
As the separator having a shutdown function, for example, a polyolefin porous film is used. At the time of abnormal heat generation in the battery, a separator consisting of the polyolefin porous film is melted and made non-porous at about 80 to 180° C. to block (shut down) passage of ions, thereby suppressing additional heat generation. However, in the case of intensive heat generation etc., a separator consisting of the aforementioned porous film may come into direct contact with each other due to shrinkage, film breakage or the like, leading to occurrence of a short-circuit. Thus, a separator consisting of a polyolefin porous film has insufficient morphological stability, so that abnormal heat generation due to a short-circuit may not be suppressed in some cases.
As a measure for improvement, some non-aqueous electrolyte secondary battery separators excellent in morphological stability at a high temperature have been proposed. As one means thereof, there has been proposed a non-aqueous electrolyte secondary battery separator consisting of a laminated porous film in which a heat resistant layer including a filler of fine particles and a porous film as a base principally including a polyolefin (hereinafter, referred to as a “base porous film” in some cases) are laminated (see, for example, Patent Document 1). In such a separator, suppressing fall-off of a filler from the surface of a laminated porous film is one of the challenges.
When the filler falls off from the separator, physical properties expected as a separator are not exhibited, or a process failure occurs such as contamination of equipment with a powder (filler) falling off at the time of assembling a battery.
When simply the amount of a binder is increased as a method for suppressing fall-off of a filler as described above, ion permeability etc. is reduced, and therefore there have been proposed a method in which the surface of a filler is modified (see, for example, Patent Document 2), a method in which characteristics are imparted to the chemical structure of a binder resin for binding a filer (see, for example, Patent Document 3), and a method in which the average fiber diameter of fibers to fix a filler and the particle diameter of the filler are controlled to have a predetermined relationship (see, for example, Patent Document 4).
However, it is difficult to say that suppression of fall-off of a filler by these methods is sufficient, and further improvement is required.
The non-aqueous electrolyte secondary battery includes a positive electrode sheet and a negative electrode sheet with a positive electrode mixture or a negative electrode mixture carried on a sheet-shaped collector, and has a structure in which the positive electrode sheet, the separator and the negative electrode sheet are laminated in this order.
Such a positive electrode sheet and negative electrode sheet (hereinafter, referred to collectively as an “electrode sheet” in some cases) may also be used as a laminated electrode sheet provided with a heat resistant layer for preventing a short-circuit at the time of abnormal heat generation in the battery.
Such a laminated electrode sheet having a heat resistant layer has the challenge of suppressing fall-off of a filler as in the case of the separator.